The objectives of this proposal are to evaluate the effects of administering bolus PEG-rHuMGDF (3 ug/kg) subcutaneously to normal human volunteers on marrow megakaryocytopoiesis, and the viability and function of the resulting platelet product. Specifically, we propose to measure the changes in marrow megakaryocytopoiesis, determine the deterations in platelet number, volume, lifespan, and turnover, evaluate the modulating effects of PEG-rHuMGDF on platelet function, and evaluate the changes in endogenous thrombopoietin (TPO) and platelet (TPO) receptor numbers.